The assert function checks whether its first argument is not false and simply returns that argument; if the argument is false (that is, false or nil), assert raises an error. Its second argument, the message, is optional, so that if you do not want to say anything in the error message, you do not have to. Beware, however, that assert is a regular function. As such, Lua always evaluates its arguments before calling the function. Therefore, if you have something like